Ultra-Thin Whitetopping, Public Roads: Vol. 62, No.2 September/October 1998

Introduction
Repairing bumpy, rutted, pot-holed pavements - although necessary - is one of the biggest inconveniences to motorists today. Especially if the pavement happens to be at a busy downtown intersection.

Several years ago, however, a new technique/process in which 50 to 100 millimeters of high-strength, fiber-reinforced concrete is placed over a milled surface of distressed asphalt concrete pavement was introduced, and this process, called "ultra-thin whitetopping" (UTW), has proven to be a low-cost, effective, and fairly simple solution. UTW is designed for low-speed traffic Cooperative Research and Developemnt Agreement between FHWA and American Concrete Pavement Association (ACPA).

Cooperative Research and Developemnt Agreement between FHWA and American Concrete Pavement Association (ACPA).

areas or areas with a lot of stop-and-go traffic, such as street intersections, bus stops, or toll booths. The best part is that UTW requires significantly less time to construct and repairs last much longer.

Given its success in these limited applications, UTW is now being considered for a range of other applications. In fact, a few states have pilot projects using UTW as an alternative to asphalt overlays for interstate roads. There are, however, still a lot of unknowns about the process. To help state and local highway agencies make decisions about using UTW for other applications, the Federal Highway Administration (FHWA) recently kicked off a research project that will take some of the risk out of making those decisions.

UTW Project Launch
On May 11, 1998, FHWA and the American Concrete Pavement Association (ACPA) launched a joint UTW research effort. The research, which is being conducted under a Cooperative Research and Development Agreement (CRADA), will evaluate critical design factors affecting the performance of UTWs.

The "launch" and the CRADA-signing ceremony occurred at FHWA's Turner-Fairbank Highway Research Center (TFHRC) in McLean, Va. FHWA's Deputy Administrator Gloria Jeff spoke at the ceremony about the opportunities that come from technology partnerships such as the UTW joint research team. FHWA's director of engineering research and development, Charles Nemmers, explained that "through this cooperative effort we will be able to take a process that has had some very successful initial field trials in a number of urban situations and develop a much better understanding of its performance in terms of standard engineering parameters."

The CRADA calls for ACPA, in cooperation with the Virginia Ready Mix Concrete Advisory Council and ACPA's Northeast Chapter, to arrange for the design of the concrete mixes and for the construction of the UTW pavement sections. FHWA, in turn, will test the material properties for all pavement layers, test the pavements with TFHRC's Accelerated Loading Facility (ALF), and provide the data for a cooperative evaluation of the design method by ACPA and FHWA.

"Through this effort, we will enhance our understanding of the performance of UTW in a range of pavement applications," said Robert Long, director/CEO of ACPA's Northeast Chapter. "In particular, testing with THFRC's ALF allows a comprehensive, objective evaluation of the design features and overall performance of this technology, which has been used in more than 200 applications across the United States since its inception in 1991."

The Accelerated Loading Facility (ALF) is a main feature of FHWA's Pavement Testing Facility, a permanent pavement testing laboratory located on the grounds of the Turner-Fairbank Highway Research Center in McLean, Va. The first of its kind in North America, ALF is used to simulate traffic loading and has the capability of simulating 20 years of traffic loading in six months or less. The ALF consists of a 29-meter-long structural frame with a moving wheel assembly. The wheel assembly models one-half of a single axle and can apply loads ranging from 44.5 to 100.1 kilonewtons. It travels 18.5 kilometers per hour over a 9.8-meter test pavement section. To simulate highway traffic, the ALF loads pavement in one direction, and the loads are laterally distributed to simulate the side-to-side wander of trucks. ALF is computer-controlled, permitting operation 24 hours per day, seven days a week. It can use truck super-single or dual tires.

ALF tests pavement sections that represent typical highway pavements and are constructed using normal highway materials, equipment, and procedures. Various instruments and equipment are available to measure load-associated responses, such as stresses, strains, and deformations; pavement performance characteristics, such as rutting, cracking, and roughness; and environmental effects, including pavement temperature and base and subgrade moisture conditions. Data acquisition and analysis are performed using personal-computer-based systems and are used to validate pavement design concepts and to provide guidance on the use of new materials in pavement construction. Sampling of Completed UTW Projects.

UTW Research Project
The specific objectives of the joint FHWA-ACPA UTW research project are to:

Evaluate UTW performance under controlled, ALF wheel loads and temperature.
Study the effects of design features on the performance of UTW.
Measure pavement response to validate mechanistic models to predict the load-carrying capacity of UTW.

To achieve these objectives, the research project involves the design, construction, instrumentation, and performance evaluation of eight 48-foot (14.6-meter) lanes of previously trafficked asphalt concrete (AC) pavements that have been milled and overlaid with UTW. The UTW overlays are 2.5 inches (63 millimeters) and 3.5 in (89 mm) with and without polypropylene fibers and have 3-ft (0.9-m), 4-ft (1.2-m), and 6-ft (1.8-m) joint spacings. The AC pavements, which were previously used to evaluate Superpave, are 8 in (200 mm) of AC over 18 in (460 mm) of crushed stone base. Prior to UTW placement, they were milled 2.5 in and 3.5 in.

During construction, FHWA conducted a series of standard concrete quality-control tests, including temperature, slump, unit weight, and air content. A range of other materials tests will also be conducted throughout the project. These include tests of compressible strength, modulus of elasticity, and flexural strength and a series of tests developed by Iowa DOT to measure UTW bond strengths. To evaluate the pavements' performance, FHWA installed 15 to 18 strain gauges in each lane near the top and the bottom of the UTW and on top of the AC. These gauges will measure deflections and strains at various locations in the concrete slabs and the overlaid AC layer. Interior and corner deflections will also be monitored with linear voltage displacement transducers (LVDTs) and lasers. Falling-weight deflectometer and laboratory tests will also be used to compute the k-value of the subgrade and the modulus of the AC.

ALF - A Key Feature of the Project
To better understand the effects of traffic loads on UTW-treated pavements, the research project is using ALF. ALF applies real-world truck loading in a controlled setting.

"ALF provides an ideal solution for this research effort," explained James Sherwood, a research highway engineer at TFHRC. "Instrumenting and monitoring in-service pavements where UTW has been used - for example, busy downtown intersections - would be virtually impossible, given the inconvenience to motorists and the safety issues involved with such an effort. With ALF, we can measure these pavements' performance characteristics consistently and accurately 24 hours a day, seven days a week."

The initial ALF wheel loads used for this study will be 10,000 pounds (4,536 kilograms) (one half of a 20,000-pound, single-axle load) on dual 11R22.5 radial ply truck tires, which will travel at 10 miles per hour (16 kilometers per hour) in a single wheel path.

Data Collection and Analysis
Over the course of the project, data will be collected on an ongoing basis. A manual procedure will be used to record data on cracks, spalls, and other surface defects in the UTW overlays. Longitudinal and transverse profiles will be measured with a "dipstick" and rod-and-level surveys before and after testing. During testing, a digital faultmeter designed by Georgia will be used to monitor performance. Strains and deflections under the ALF rolling loads will be measured at set intervals - or when significant distress appears - until the test ends. Analyses of the data collected throughout the project will focus on validating current UTW pavement design concepts in order to provide guidance on the use of this new material in pavement construction and rehabilitation.

Project Time Line
UTW construction was completed April 7, 1998, and the first load application occurred during the project launch on May 11. The estimated duration of each of the eight tests is from four to 13 weeks. The overall duration of the study is 18 months.

As with any basic research effort, its benefits are reflected largely in the information it provides. Although UTW has proved promising to date, the key to its success is the development of appropriate design guidelines for agencies and pavement engineers. This project will provide the basic data needed to effectively assess UTW's performance in a range of settings. The ultimate goal of the project is to gain enough knowledge about UTW to be able to develop guidelines that will assist engineers and reduce any risk in using UTW in day-to-day operations.

is the chief of FHWA's Pavement Performance Division, Office of Engineering Research and Development. The division is responsible for several of FHWA's key pavement research initiatives: the Long Term Pavement Performance Program, performance-related specifications for asphalt and portland cement concrete pavements, and truck-pavement interaction. He began his highway career with the Pennsylvania Department of Transportation in 1965 and joined FHWA in 1981 as a geotechnical engineer. Prior to becoming division chief, he served as the Strategic Highway Research Program Implementation Coordinator in FHWA's Office of Technology Applications.